Magnesium oxide is a compound excellent in heat resistance, thermal conductivity, and electric insulation and is widely used in various industrial fields such as a vulcanization accelerator for rubber, pigment for paint and ink, and drugs. Heat-releasing filler has been proposed as one of the various applications of such magnesium oxide (Patent Documents 1, 2, etc.).
Alumina, aluminum nitride, etc. are commonly used today as such a heat-releasing filler. However, there is a problem that a kneading machine is worn very much in the course of the production of a heat-releasing sheet, etc. because of the high Mohs hardness of alumina. In addition, aluminum nitride has a drawback that its filling into resin in a high content is difficult to attain because of its poor filling property. Moreover, aluminum nitride also has a drawback that it raises the price of heat-releasing members because of its high expense. Therefore, there is a demand for new heat-releasing fillers different from these raw materials.
On the other hand, magnesium oxide particles have an advantage of excelling in handleability because the particles are made of a compound low in Mohs hardness and low in specific gravity. Moreover, because of being a material with a high electric resistance value, magnesium oxide particles are suitable also for use in the electrical and electric fields. However, magnesium oxide has a drawback of being poor in water resistance and acid resistance. For this reason, there is a problem that under usage conditions such that magnesium oxide comes into contact with water, it is transformed into magnesium hydroxide, so that durability against water cannot be obtained. Moreover, there is another problem that expansion occurs upon conversion into magnesium hydroxide due to water absorption.
On the other hand, Patent Documents 3 and 4 are known as a method of coating the surface of an inorganic powder with an epoxy resin. The coating with an epoxy resin disclosed in these documents, however, is coating formed by dispersing it in an aqueous medium. For this reason, there is a problem that if such treatment is applied to magnesium oxide, the magnesium oxide is degenerated due to hydration and transformed into magnesium hydroxide. Therefore, magnesium oxide coated with an epoxy resin cannot be obtained by such a method.